Nintendo Entertainment SystemRenderingAdvanced

Sprite 0 Split

Keep a status bar fixed at the top of the screen while the playfield below scrolls — by placing sprite 0 at the boundary and changing the scroll the instant its hit flag fires. The NES's answer to the mid-screen split.

ppusprite-0split-screenscrollingstatus-bar

Overview

Scroll is set once per frame, so the whole screen scrolls together — yet almost every NES game has a fixed status bar: score, lives, a map, sitting still above a scrolling world. The PPU has no scroll-per-line register, so the trick is timing. The hardware raises a sprite 0 hit flag the moment sprite 0’s first opaque pixel is drawn over an opaque background pixel. Park sprite 0 on the boundary line, keep the top of the frame at scroll zero, and the instant that flag fires — mid-screen — rewrite the scroll for the playfield below. One flag, watched by the CPU, turns a single-scroll machine into a two-zone display. It’s the NES cousin of the C64 raster split and the Amiga copper split — same effect, three very different mechanisms.

Code

; =============================================================================
; SPRITE 0 SPLIT - NES
; Top: fixed status bar (scroll 0). Below: the scrolling playfield.
; The main thread watches sprite 0 hit and switches scroll at the boundary.
; =============================================================================

PPUCTRL   = $2000
PPUSTATUS = $2002
PPUSCROLL = $2005

.segment "ZEROPAGE"
pf_scroll_x:  .res 1       ; the playfield's scroll (set by game logic)
pf_nametable: .res 1

.segment "CODE"

; --- Call once per frame, after the NMI has set the status bar at scroll 0 ---
do_split:
    ; 1. wait until LAST frame's sprite 0 hit has cleared
:   bit PPUSTATUS          ; copies bit 6 (sprite 0 hit) into the V flag
    bvs :-                 ; still set? keep waiting

    ; 2. wait until THIS frame's hit fires — the beam has reached the boundary
:   bit PPUSTATUS
    bvc :-

    ; 3. we're on the split line: hand the lower screen the playfield scroll
    bit PPUSTATUS          ; reset the write latch
    lda pf_scroll_x
    sta PPUSCROLL          ; horizontal
    lda #0
    sta PPUSCROLL          ; vertical (the playfield starts at its own top)
    lda #%10000000
    ora pf_nametable
    sta PPUCTRL
    rts

Trade-offs

Aspect Cost
CPU The main thread busy-waits on the hit flag — a slice of every frame burned
Memory Sprite 0 reserved on the boundary; a few bytes of state
Limitation Only one hit per frame — one split; and sprite 0 must be positioned exactly

When to use: A fixed HUD over a scrolling playfield — the standard NES game layout.

When to avoid: Full-screen scroll with no panel — and games with a mapper that provides scanline IRQs (MMC3), which give steadier splits without burning CPU on the poll.

Making the hit happen

Sprite 0 hit only fires under specific conditions, and each is a classic bug when missed:

  • Opaque over opaque. Sprite 0’s non-transparent pixel must overlap a non-transparent background pixel. A sprite 0 sat over blank sky never triggers — put a solid background tile under it.
  • Rendering on. Both background and sprites must be enabled in PPUMASK; the hit is a rendering side effect.
  • Not at the far edge. The hit can’t be detected at x=255, so keep sprite 0 clear of the extreme right.

Place sprite 0’s bottom on the last line of the status bar, and the flag fires just as the beam finishes the bar and reaches the playfield.

Two waits, then the switch

The poll is two-phase, and the order matters. First wait for the hit flag to be clear — otherwise last frame’s stale hit fires the split instantly, at the top. Then wait for it to set. bit PPUSTATUS is the whole idiom: it drops bit 6 into the 6502’s overflow flag, so bvs/bvc branch on the hit directly, no masking. When the second wait falls through you’re on the boundary line — reset the latch and write the playfield’s scroll, and everything below renders shifted while the bar above stayed put.

The cost, and the alternative

This works on any NES, mapper or not — but it spends the CPU waiting for the beam, often a good fraction of the frame, and the split can jitter a scanline if the poll loop is interrupted. Games built on the MMC3 mapper skip the poll entirely and use its scanline counter IRQ to fire an interrupt at the chosen line — steadier, cheaper, and able to split the screen more than once. Sprite 0 is the universal method; the mapper IRQ is the professional one.

Patterns: Nametables and Scrolling, Feeding New Columns

Vault: PPU | Nintendo Entertainment System